1. Field of the Invention
The present invention relates to an input matching circuit connected to an input circuit of a low noise amplifier which is used in a converter for direct broadcast satellite (DBS) systems.
1. Related Background art
A noise figure (NF) of an amplifier which uses a field effect transistor (FET) varies with a signal source impedance viewed from the FET toward a signal source, and it is minimum at a certain signal source impedance (Z.sub.opt). Thus, an input matching circuit is provided in the signal source of the FET so that the signal source impedance viewed from the FET assumes an optimum signal source impedance Z.sub.opt.
However, the optimum signal source impedance Z.sub.opt varies with variations in the manufacture of the FET. A prior art input matching circuit of the low noise amplifier has been designed such that the signal source impedance viewed from the FET is a mean value of the variations of the optimum signal source impedance Z.sub.opt.
However, a constant noise contour on a Smith chart for an FET is not symmetric to the optimum signal source impedance Z.sub.opt but rather is offset therefrom. As a result, even if an external impedance (50.OMEGA.) is converted to the mean value of the optimum signal source impedance Z.sub.opt, a mean value of variations of the NF of the amplifier is not necessarily minimum due to the offset of the constant noise contour.